Washington — With Ukraine moving to the brink of open confrontation with Russia, and Moscow ordering fresh military exercises on the countries' common border, Secretary of State John Kerry issued a blunt warning to Russian President Vladimir Putin Thursday evening: Reverse his course of “provocation” or face “grave” and “costly” consequences from a united international community.

"The window to change course is closing,” said Secretary Kerry, as he laid out a list of covert and destabilizing actions he said Russia has pursued against Ukraine over the past week.

“If Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake," he concluded.

Mr. Obama, at the start of a four-nation tour of Asia, raised the prospect of tougher economic sanctions from the West and said, “We are ready to act.” He was responding to comments from Mr. Putin Thursday warning the Ukrainian government that it faces “consequences” for moving against pro-Russia separatists in eastern Ukraine.

Appearing to presage deeper Russian intervention, Putin said Russia would “have to react to such developments.” Ukrainian security forces attempting to clear roadblocks and take back barricaded government buildings in eastern Ukraine killed as many as five separatists Thursday.

Kerry’s statement, the most detailed and direct of those coming from US leaders, sounded like a final warning shot.

Kerry said Russia’s destabilizing activities and thinly disguised covert actions in Eastern Ukraine had started even as the US, Russia, the European Union, and Ukraine were meeting in Geneva last week to forge a stabilization plan to end the Ukraine standoff. But while the government in Kiev was doing everything it had agreed to under the plan, Kerry said, Russia was stepping up activities designed to undermine and provoke the Kiev government.

Offering copious examples of the subversion he was referring to – disciplined paramilitary groups in new but unmarked uniforms occupying Ukraine government buildings, arrests in Ukraine of undercover Russian intelligence agents, the kidnappings and murders of local Ukrainian officials and journalists – Kerry said, “We’ve seen this movie before – mostly recently in Crimea.”

Insisting that the world is not being fooled by Russia’s “propaganda” and “President Putin’s fantasy” of peaceful but besieged Russian-speaking protesters, Kerry said, “Russia is stoking the very instability they say they want to quell.”

Putin and other Russian officials have mocked the impact of existing or potential Western sanctions on the Russian economy. But Kerry offered statistics suggesting the Russian economy is already hurting from sanctions imposed over the Crimea annexation. He also cited some Russian officials acknowledging the blow sanctions have already struck, in particular to foreign investment and the confidence of foreign investors in considering Russia in the current climate.

But some analysts are considerably less confident than Kerry appears to be that the “international community” will be united in imposing a new round of broader sanctions on sectors of the Russian economy.

Experts considering sanctions at a Washington forum sponsored by the Center for Strategic and International Studies Thursday underscored that Western Europe remains reliant on Russian energy and is likely to find it difficult to replace that energy in time for next winter.

Others note that the European Union is not united on imposing broad sector sanctions, and that some Europeans could be sympathetic to a Russian claim, if it moved into Ukraine, that it acted to restore order where a civil war threatened.

“I can imagine the Russians claiming they acted to stop the unrest and reestablish order, and if they take that to the Germans and maybe some others, that could fall on some sympathetic ears, and we could see some breaks in a united Western front,” says Nikolas Gvosdev, a specialist in Russian security affairs at the Naval War College in Newport, R.I. “That would put Obama in a tough spot.”